The success rates in patients undergoing In Vitro Fertilization (IVF) has improved tremendously over the past few years. Better ovulation induction protocols as well as great improvements in the IVF laboratory have greatly contributed to the increased pregnancy rate. One area that has received more attention than usual is the embryo transfer technique. While many variations exist in the preparation of the cervix, the type of transfer catheter used, the duration of the transfer and the use of trial transfers, most physicians in the United States as well as other countries are now performing embryo transfers under ultrasound guidance. Since the introduction of the ultrasound as an aid to embryo transfer, physicians have reported a greater ease of transfer, and more accurate placement of the embryos within the uterine cavity. The patients also enjoy watching their embryos, distracting them from the actual transfer procedure.
Ultrasound guided embryo transfer has been around since 1985, however it has become almost universal in the past three years. Since then, multiple studies have shown higher pregnancy rates when performing embryo transfers in conjunction with ultrasonography (1-4). The advantages of ultrasound guided embryo transfers include the facilitation of embryo transfer as well as physician's ability to visualize catheter and embryo placement.
Although 2D ultrasound guidance has been increasingly utilized, controversy exists regarding the ideal placement of the embryos within the uterine cavity. Various studies have been published in regards to the ideal area of embryo transfer. However these studies only represent generalized locations, not taking into consideration that uterine anatomy varies among women.
Integral to a successful in vitro fertilization (IVF) is the placement of the embryo in the uterine cavity. The transfer of an embryo from the laboratory to the uterus is generally referred to as embryo transfer. Embryo transfer is generally the final step in an IVF cycle, and a successful embryo transfer is a prerequisite for achieving pregnancy. While the IVF technology and success rates have come a long way, there still remains room for improvement. The pregnancy rate is highly influenced by the quality of the embryos, the receptivity of the endometrium and by the transfer technique (5).
Because of the importance of the embryo transfer phase, efforts have been made since the introduction of IVF approximately twenty years ago to determine the “best spot” within the uterine cavity to implant the embryo. Two dimensional (2D) sonography has been used for some time to aid in embryo transfer. While this represented an advance over implanting embryos without any guidance whatsoever, two dimensional sonography has still only provided guidance as to the general area at which the embryo should be released for implantation. There is still no consensus of what the ideal choice for implantation is for embryos. In a study by Baba et al, embryos were transferred to the midfundal area in 60 patients. Among the 22 pregnancies, 80% of the embryos implanted in the areas to which they were transferred, while 20% implanted in other areas (6).
Other studies have been published suggesting various locations within the uterine cavity where embryos should be released. These studies have suggested implantation in locations ranging from the lower uterine segment, to various distances (0.5 cm-2.0 cm) from the uterine fundus. However, these distances still only represent generalized locations. Furthermore, the value to place on these distances as guidelines decreases when considering that the configuration and dimensions of the uterine cavity vary between women. The location of embryo transfer in respect to the uterine anatomy also varies among physicians. Some studies have shown that the optimal location for embryo transfer is 0.5-1 cm away from the uterine fundus (3), while some believe that the tip of the catheter should be 1.5 cm from the uterine fundus (2). However, others have shown that transfer should be in the lower to middle uterine segment (7). In a different study by Pope et al, it was demonstrated that for every additional millimeter the embryos are placed away from the uterine fundus, the clinical pregnancy rate increases by 11% (8). In a recent randomized study by Franco et al, embryos were deposited in the lower or upper half of the endometrial cavity and there was no difference in pregnancy or implantation rates (9).
In sum, two dimensional sonography does not provide the clinician with an optimal view of where the embryo should be released. Accordingly, there exists a need for a more accurate system with which to determine the optimal location at which to implant an embryo. There also exists a need for embryo transfer to be carried out with less trauma to the woman undergoing IVF.